This invention relates generally to apparatus and a method for generating and directing high intensity x-rays. More particularly, it relates to apparatus for producing and directing such high intensity x-rays to an object of interest from any of a plurality of preselected points spaced from that object. More specifically, the invention relates to such apparatus which is suitable for use in a computer tomography (CT) x-ray scanner having no moving parts.
Conventional high intensity x-ray scanning equipment, such as is used in conventional computer tomography scanners currently available, generally incorporate one or more x-ray sources and detectors which are mechanically rotated together about an axis generally extending horizontally through the apparatus and through the object of interest. However, since the x-ray producing and detecting apparatus which must be rotated is relatively large and heavy with the source and the detecting apparatus being spaced generally several feet apart, the accelerating, centrifugal, and braking forces which must be exerted to rotate and stop the apparatus for rapid and accurate repositioning at numerous points are necessarily great and apply large stresses to both the cathode and the target assemblies of the x-ray generating tube. Additionally, the commonly used rotating target assemblies further experience large gyroscopic forces. These forces produce design difficulties and service life limitations on the apparatus. Further problems associated with these mechanically moving systems arise from the motor drive equipment, moving interconnections, slip ring assemblies, and other mechanical elements, especially with the continuing demands for increasing scan speeds and higher beam powers. Thus, when seeking further performance improvements, the conventional mechanical drive systems which must be designed to move rapidly and to accurately position the x-ray tube and its associated apparatus present severe inertial limitations and require substantial performance compromises.
The obvious approach to take to obtain further performance improvements in scan speed while avoiding these inertial limitations is through the use of an electronically scanned system which has no moving parts to present such limitations. However, the physical properties of an electron beam suitable for the production of such x-rays, and the interaction of such a beam with the scanning elements present a number of nonobvious problems which heretofor have defied solution and have prevented the development of an operable no-moving-part x-ray scanner of sufficient power to be useful, especially for computer tomography applications. The primary problems encountered have been those imposed upon focal source geometry by the space charge forces of the electron beam used to produce the x-rays and the field aberrations introduced into the beam by the beam deflecting elements. The only prior art patent known to the inventor which suggests such an electronically scanned x-ray system, Oldendorf U.S. Pat. No. 3,106,640, fails completely to address these major problems and thus, despite its relatively low energy (30 kv) electron beam, is of questionable usefulness and operability as disclosed in that patent.
Consideration of the case of a high current, high voltage x-ray scanner, such as might be used for computer tomography and which incorporates an annular or ring target with an electron beam deflected to that target from an initial trajectory along the axis of the ring, is useful for illustrating the problems necessary for solution to obtain an operable electronically scanned x-ray system. For such a case a straightforward analysis of the forces acting upon the electron beam shows that, for the required practical values of beam power, drift distance and x-ray focal source dimensions, the concept of simply deflecting the electron beam onto a large diameter ring target is not feasible for several fundamental reasons, one of the most important being the spreading of the beam due to space charge defocussing forces.
An initially parallel beam of electrons will, in free space, become increasingly divergent due to the presence of the inherent space charge of the electrons. The magnitude of these beam-spreading space charge forces at any point along the beam trajectory is a function of the electron beam energy, current and geometry. Neglecting the presence of ions, for a steady flow of electronic charge of constant density (as may be obtained with a direct current electron beam drifting in free space) the distance (z) that an initially parallel beam of radius r.sub.O can drift before space charge forces have caused the beam to diverge to a larger radius r is given by the following relativistically corrected universal beam spreading formula: EQU z = r.sub.o /.sqroot.i (.gamma..sup.2 -1) .sup.3/4 G in MKS units.
where i = electron beam current in amperes
.gamma. = energy of the electrons in rest mass units PA1 c = velocity of light PA1 and x = (ln r/r.sub.o).sup.1/2 -- a factor representing the geometric expansion of the beam. PA1 beam energy -- 130 kilovolts (kv); PA1 beam currents -- 250 to 500 mA; PA1 drift distance from beam deflector to x-ray ring target -- 2 to 2.5 meters.
and ##EQU1## where m.sub.o /e = ratio of the electron rest mass to charge .epsilon..sub.o = permittivity of free space
The space charge modified behavior of the more practically encountered non-uniform charge density beams require rigorous three dimensional analyses of the variations of charge density, beam energy and beam geometry with distance traveled. The beam core potential depression and reduced space charge effects due to the presence of surrounding walls also have to be taken into account. Such analysis may be found in Haimson and Mecklenburg, A Relativistically Corrected Three Dimensional Space Charge Analysis of Electron Bunching, IEEE Transactions on Nuclear Science, June 1967, pp. 586-93. However, for purposes of illustration, the simplified equation given above may be used with following desirable parameters for an advanced, electronically scanned x-ray CT scanner:
This drift distance is chosen to provide a vacuum chamber suitably large to comfortably encompass a patient undergoing computer tomography and to ensure sufficient traversal of the patient's longitudinal axis to permit a whole body scan procedure. By utilizing these parameters the above-described analysis requires that, to achieve an x-ray focal source diameter of 1 mm, the initial beam diameter must be larger than that by a factor of 100. Such adverse beam geometry would require a large and cumbersome drift space housing and special electron beam and gun optics and would substantially aggravate the beam aberration effects imposed upon the beam by the deflecting elements. These aberration effects also impose fundamental restrictions on achieving a narrow focal source on a large diameter ring target when using a simple deflection system (such as disclosed by Oldendorf) and a high current electron beam, as is required for computer tomography applications. Such a simple deflection system as disclosed in Oldendorf also requires the use of an undesirably large and long drift space housing due to its conical beam path between the deflection coil and the target.
To overcome the space charge limitations upon the system, conventional practice in the design of other types of electron beam devices has been to provide a simple axial magnetic field to enable the beam cross-section to be controlled by compensating for the divergence caused by the space charge forces. However, the use of such conventional extended solenoid coils located outside the vacuum chamber would be most undesirable in a high intensity computer tomography scanner for several reasons. Not only would the partial compensation of the deflection forces have to be accounted for, but, more importantly from the x-ray production and detection point of view, the electron beam so controlled would strike the target with very large azimuthal velocity components (in a direction circumferential to the ring target). For systems which incorporate radially oriented x-ray collimators in order to improve detection and to minimize oblique absorption and penumbral contribution to the patient's total integrated dose of radiation, these problems result in a substantial loss of x-ray intensity for a given electron beam energy.
Another serious limitation encountered by the use of a simple deflection system to deflect a high current electron beam onto a large diameter annular target is that the minimum size and desired shape of the focal source on the target are strongly influenced by aberrations introduced by the beam deflection system and by any other electromagnetic field elements located between the deflection system and the electron beam source. Very small initially introduced beam aberrations are highly amplified due to the long drift distance and large deflection which is necessary for such a CT system. Even if the effects of space charge beam spreading and finite emittance of the beam source are neglected, the requirement to reproduce a given focal source geometry to within, for example, 1 mm at the target, demands both an extremely high quality characteristic of the beam entering the deflector and an extremely high quality, homogeneous deflection field which is free of higher order aberrations, regardless of the size of the beam and the varying position of the beam in the field during traversal of the deflector.
Even if these various aberration effects could be individually neutralized for a given position on the target, (i.e., a specific deflection field configuration) the other azimuthal locations of the focal source, which require different deflection field configurations, would interact with the imperfect entry beam in a non-linear fashion to produce space- and time-dependent aberrations at the focal source. The degree of difficulty in overcoming these focal source distortions is increased even further by the need for designing an x-ray scanner for a CT system in which the electron beam can be switched rapidly, in microseconds, not just from one focal source position to the next contiguous location on the target, a few millimeters away, but to another focal source position that may be almost diametrically opposite and then back across to the opposite side of the patient again. In such a system, the detectors must also be gated in synchronism with the fast switching electron beam locations.